Electrical insulating paper containing polymethylol phenol



United States Patent ELECTRICAL INSULATING PAPER CONTAINING POLYMETHYLOLPHENOL Donald K. Pattilloch, New York, N.Y., assignor to MichiganResearch Laboratories, Inc., Long Island City, N.Y., a corporation ofMichigan, and Electro-Chem Fiber Seal Corporation, New York, N.Y., acorporation of Delaware No Drawing. Application May 1, 1957 Serial No.656,219

16 Claims. (Cl. 162-138) This invention relates to electrical insulatingpaper, and more particularly paper containing a polymethylol phenol.

Prior art electrical insulating paper has been prepared by impregnatingpaper with a resin or insulating varnish. It is among the objects of thepresent invention to prepare electrical insulating paper directly bypaper-making techniques and avoid the added steps of impregnating orcoating and subsequent drying or curing of the impregnated paper.

The method of the invention generally comprises reacting cellulosefibers while suspended in an aqueous medium with a water solublepolyaldehyde, reacting the resulting reaction product with apolymethylol phenol and forming the suspension into a sheet. by usualpaper-making techniques and the polymethylol phenol is cured to anisoluble resinous form in the drying step. The paper of the presentinvention exhibits high insulating qualities, having an electricalbreakdown strength in excess of 300 volts per mil of sheet thickness.The paper also exhibits excellent dimensional stability, high tensilestrength and 50% higher bursting strength (Mullen strength) thanconventional paper prepared from the same pulp stock. The paper may beused as electric motor slot insulation, wrappers for electrical cableand other applications.

The water soluble polyaldehyde reacts with the hydroxyl groups of thecellulose and leaves one or more reactive aldehyde groups free to reactwith the polymethylol phenol to link the same to the cellulose. Thepolyaldehyde must have two or more aldehyde groups in the molecule.Suitable Water-soluble polyaldehydes include:

Glyoxal a-Hydroxyadipaldehyde Glutaraldehyde Succinaldehyde Z-ethylglutaraldehyde B-methyl glutaraldehyde Z-ethyl 3-methyl glutaraldehydeThe polymethylol phenol useful in the present invention is a phenolhaving two or more methylol groups 2,4-dimethylol phenol and2,6-dimethylol phenol. The compounds are water soluble monomers preparedby careful condensation of formaldehyde and phenol to avoid polymerformation. Their water solutions are alkaline and may contain a smallpercentage of free formaldehyde, which does not interfere in thereaction of the in- The paper is formed 2,917,425 Patented Dec. 15, 1959vention. The polymethylol phenols are cured to an insoluble resin bysuitable drying temperatures.

Example I Examples II-VIII Unbleached kraft pulp was beaten asin ExampleI with varying percentages of poly aldehydes in the following examples.Thereafter varying percentages of 2,4, 6-trimethylol phenol were added,beating continued, and hand sheets formed as in Example I. Theelectrical breakdown strength of the hand sheets formed from the stocksis set forth in the following table.

Percent Break- Ex. Polyaldehyde TMP down,

volts/mil 2% glyoxal 10 324 5% glutaraldeivde 5 314 5%m-hydroxyadipaldehyda- 5 308 3% glyoxal 10 336 do 3 306 10 329 5%glyoxal 10 342 TMP in the above table refers to 2,4,6-trimethylolphenol.

The insulating paper of the instant invention is conveniently made onconventional paper machines, including Fourdrinier and cylindermachines, using conventional techniques. Machine run paper of theinvention, having a higher and more uniform density than hand sheets,shows electrical breakdown resistance in the range of 420-500 volts permil.

It has been found that 2-5 of polyaldehyde and about 5-10% ofpolymethylol phenol produce electrical insulating paper of good quality.Higher percentages may be used but generally are uneconomical. Thecellulose pulp may be of any type, bleached or unbleached, such askraft, sulfite, soda or semichemical pulp. For most electricalinsulating paper applications, unbleached kraft pulp is preferred.

Under certain operating conditions, it may be necessary to augment thenormal drying cycle of the paper machine by the use of auxiliary dryingequipment, such as infrared banks.

All percentages are by weight on the basis of added solids on fibersolids.

While the invention has been described in terms of certain examples,such examples are to be considered illustrative rather than limiting,and it is intended to cover all modifications and embodiments that fallwithin the spirit and scope of the appended claims.

What is claimed is:

1. The process for the preparation of electrical insulating paper whichcomprises reacting cellulose fibers while suspended in an aqueous mediumwith a water-soluble polyaldehyde, reacting the resulting reactionproduct with a polymethylol phenol, and forming the suspension into asheet.

2. The process set forth in claim 1 wherein the polymethylol phenol is2,4,6-trimethylol phenol.

3. The process set forth in claim 1 wherein the watersolublepolyaldehyde is glyoxal.

4. The process set forth in claim 1 wherein the water solublepolyaldehyde is glutaraldehyde.

5. The process set forth in claim 1 wherein the water solublepolyaldehyde is a-hvdroxyadipaldehyde.

, 6. The process for the preparation of insulating paper which comprisesreacting cellulose fibers while suspended in an aqueous medium withglyoxal, reacting the resulting reaction product with 2,4,6-trimethylolphenol, and forming the suspension into a sheet.

: 7. Insulating paper comprising a cellulosic web, the individual fibersof which comprise cellulose fibers reacted with a water-solublepolyaldehyde and the reaction product thereof reacted with apolymethylol phenol.

8. Paper set forth in claim 1 wherein said water soluble polyaldehyde isglyoxal.

9. Paper set forth in claim 7 wherein said polymethylol phenol is2,4,6-trimethylol phenol.

10. Paper settforth in claim 7 wherein said watersoluble polyaldehyde isglutaraldehyde.

11. Paper set forth in claim 7 wherein said watersoluble polyaldehyde isa-hydroxyadipaldehyde.

12. Insulating paper comprising a cellulosic web, the individual fibersof which comprise cellulose fibers reacted with glyoxal and the reactionproduct thereof reacted with 2,4,6-trimethylol phenol.

13. Insulating paper comprising a cellulosic web, the individual fibersof which comprise cellulose fibers reacted with glutaraldehyde and thereaction product thereof reacted with 2,4,6-trimethylol phenol.

14. Insulating paper comprising a cellulosic web, the individual fibersof which comprise cellulosic fibers reacted with a-hydroxyadipaldehydeand the reaction prod,- uct thereof reacted with 2,4,6-trimethylo1phenol.

15. The process for the preparation of insulating paper which comprisesreacting cellulose fibers while suspended in an aqueous medium withglutaraldehyde, reacting the resulting reaction product with2,4,6-trimethylol phenol, and forming the suspension into a sheet.

16. The process for the preparation of insulating paper which comprisesreacting cellulose fibers while suspended in an aqueous medium witha-hydroxyadipaldehyde, reacting the resulting reaction product with2,4,6-trimethylol phenol, and forming the suspension into a sheet.

References Cited in the file of this patent UNITED STATES PATENTS2,037,522 Lundback Apr. 14, 1936 2,285,490 Broderick June 9, 19422,495,232 Drisch et al. Jan. 24, 1950

1. THE PROCESS FOR THE PREPARATION OF ELECTRICAL INSULATING PAPER WHICHCOMPRISES REACTING CELLULOSE FIBERS WHILE SUSPENDED IN AN AQUEOUS MEDIUMWITH A WATER-SOLUBLE POLYALDEHYDE, REACTING THE RESULTING REACTIONPRODUCT WITH A POLYMETHYLOL PHENOL, AND FORMING THE SUSPENSION INTO ASHEET.